Rivet with sliding cap for friction stir riveting

ABSTRACT

A friction stir rivet is rotated and driven through a first fusible workpiece into an engaged second fusible workpiece, causing local portions of the first and second workpieces to plasticize. A slideable cap contacts the exposed surface of the first workpiece shortly after the process begins. The contact causes the cap to act as a retaining element, limiting the escape of plasticized material from stir site. Once the rivet is driven into the first and second workpieces, rotation ceases and the plasticized material hardens around the rivet. A weld is thus created, joining the workpieces and encompassing the rivet, which provides additional mechanical strength.

The present application is a divisional application of patentapplication Ser. No. 10/337,017 which was filed on Jan. 6, 2003 now U.S.Pat. No. 6,883,699, and which is incorporated herein in its entirety.

TECHNICAL FIELD

This invention relates to friction stir welding and riveting, moreparticularly, to methods of joining multiple workpieces using a stirrivet to create a mechanical weld, an interweld, and a diffusion bond.

BACKGROUND OF THE INVENTION

Friction stir welding (FSW) is a method used to join metal workpieces.The method generally uses a cylindrical, shouldered tool with a profiledpin that is rotated at the joint line between two workpieces while beingtraversed along the joint line. The rotary motion of the tool generatesfrictional heat which serves to soften and plasticize the workpieces.This softened material, contributed by both workpieces, intermingles andis consolidated by the pin shoulder. As the pin moves laterally thefrictional heating is reduced and the softened material hardens,creating a bond between the two workpieces. The best currentunderstanding of the process is that no melting occurs and the weld isleft in a fine-grained, hot worked condition with no entrapped oxides orgas porosity.

Stir rods used in conventional FSW are typically symmetrical cylindershaving an enlarged fixed cap located on their upper side. The fixed capused in conventional FSW does not engage a workpiece until the end oftool insertion, allowing a majority of the initially plasticizedmaterial to be expelled from the cavity before the cap creates a sealaround the worksite. Current methods used in FSW do not teach or suggestmethods of engaging a cap and a workpiece at the beginning of theprocess to retain the maximum amount of plasticized material in the weldzone.

SUMMARY OF THE INVENTION

This invention is based on a newly developed method which we callfriction stir riveting. This method improves friction stir welding byusing a stir rivet having a slideable cap. The stir rivet is rotated andadvanced into a pair of workpieces to plasticize material around therivet for stir welding the workpieces together. Near the beginning ofthe process, the slideable cap contacts the first workpiece. The contactbetween the cap and the first workpiece creates a partial seal, limitingthe amount of plasticized material displaced out of the stir site. Therivet is then left in place to form a weld between the rivet and thesolidified material.

The present invention utilizes a friction stir rivet having a bodyincluding an elongated cylindrical section and upper and lower stops atopposite ends of the cylindrical section. The cylindrical section of thebody extends through a cap. A spring may extend between the cap and theupper stop, or the cap and a driving apparatus. An interlocking guideextends longitudinally along a portion of the cylindrical section. Theinterlocking guide on the cylindrical section may be a flat surface.

The cap has a central opening surrounding the cylindrical section. Thecentral opening of the cap has an interlocking guide compatible with aninterlocking guide of the cylindrical section, which causes the cap torotate with the body. The interlocking guide in the central opening ofthe cap may be a flat surface. Alternatively, a threaded surface may beused to form a guide between the cylindrical section and the opening ofthe cap.

The upper stop forms the head of the rivet and provides a physicalbarrier, which can be used to compress the spring against the slideablecap, biasing the cap toward the lower stop. If the upper stop is notused to compress the spring, a retainer located on a rotary driveapparatus can compress the spring against the cap, biasing the capagainst the lower stop. The upper stop limits upward travel of the cap.

The lower stop limits downward travel of the cap. The underside of thelower stop forms a lower end of the rivet which contacts the workpiecesto be joined. The lower stop may be applied or formed after the cap isslid over the cylindrical section of the rivet. Once the cap is on thecylindrical section of the body the lower stop can be created or appliedin any suitable manner, such as peening the lower end of the cylindricalsection, deforming the lower end, pinning the lower end to act as thestop or to secure separate stop member, or by enlarging the lower end ofthe rivet with extra material.

A recessed socket is centrally located on the upper portion of the upperstop and is aligned with the rotational axis of the rivet. To rotate therivet, a rotational rotary device is inserted into the recessed socketof the rivet.

The rivet, when rotated, locally softens and penetrates the workpieces,creating a cavity filled with plasticized material. Shortly after thelower end of the rivet penetrates the first workpiece, the slideable capcontacts the first workpiece to create a seal around the stir site,thereby limiting the amount of plasticized material displaced out of thecavity, ensuring that the plasticized material fills the cavity, andpromoting intimate contact between the rivet and the plasticizedmaterial

As the rivet advances into the workpieces, the cap slides up thecylindrical section of the rivet toward the upper stop, while the biasof the spring continues to press the cap against the first workpiece.

Upon reaching a desired depth, the rotary motion is stopped and the stirsite is cooled to provide an internally welded joint maintained togetherpartially by the shape of the rivet and partially by the welding of thecomponents together.

Preferably, the cylindrical section of the rivet body has a smallerradial thickness than the lower stop to create a re-entrant portionalong the cylindrical section. Alternatively, threads on the cylindricalsection may be used to create re-entrant portions along the cylindricalsection of the body. The re-entrant portion allows plasticized materialto fill in above the lower stop, thereby, increasing the mechanicalretention of the rivet in the workpieces.

The slideable cap limits oxygen access to the rivet during the stirringprocess by creating a seal between the rivet and the first workpiece.The reduced oxygen supply around the rivet reduces the formation ofoxides on the body of the rivet. Reducing oxidation allows a better bondto form between the rivet and the workpieces.

The rivet should be formed of a relatively high melting point metal orrefractory metal so that the rivet has a higher melting point than theworkpieces to be joined. Preferably, the rivet should have a meltingpoint that is at least 100° Fahrenheit higher and more preferably atleast 200° Fahrenheit higher than workpieces, such as aluminum. Further,the rivet should be formed of a metal of substantially greater hardnessthan the metal workpieces to be joined. Exemplary metals include highcarbon steel, titanium (e.g. titanium 6–4) and the like. Preferably, therivet should be formed of a metal that is capable of forming a diffusionbond with the metal workpieces to be joined.

A driving apparatus is used to rotate and press the rivet into the metalworkpieces to be joined. The rivet penetrates best when it is rotated atspeeds between 4,500 and 27,000 revolutions per minute. The amount ofpressure needed to allow the rivet to penetrate the metal workpiecedepends upon the speed of rotation. The rate of penetration is increasedwhen the amount of pressure applied is increased, or when therevolutions per minute are increased. Under good conditions, a frictionstir rivet can penetrate aluminum at up to 27 millimeters per minute.

The foregoing description is directed, as an example, to joiningaluminum metal workpieces with a stir rivet made of metal with a highertemperature melting point. However, it should be understood that otherfusible materials may be joined using the same process with a properselection of compatible materials. Thus, other metals and thermoplasticsmay also be successfully joined with a stirring rivet and process withinthe guidelines above described.

These and other features and advantages of the invention will be morefully understood from the following description of certain specificembodiments of the invention taken together with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a side view of an exemplary embodiment of a friction stirrivet according to the invention;

FIG. 2 is a cross-sectional view from the line 2—2 of FIG. 1;

FIG. 3 is a cross-sectional view showing the friction stir rivet of FIG.1 at the conclusion of rotation during stir riveting of two workpiecestogether;

FIG. 4 is a cross-sectional view showing the combination of analternative embodiment of friction stir rivet with associated rotarydrive and biasing apparatus; and

FIG. 5 is a cross-sectional view showing the combination of FIG. 4 atthe conclusion of rotation of the rivet during stir riveting of twoworkpieces together.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIGS. 1 and 2 of the drawings in detail, numeral 10generally indicates a friction stir rivet 10. Rivet 10 includes anelongated body 11 having a cylindrical section 12 with enlarged upperand lower stops 14, 16 at opposite ends of the cylindrical section 12.The cylindrical section 12 extends through a cap 18 and a spring 20. Aninterlocking guide 22 extends longitudinally along the cylindricalsection 12. Preferably, the interlocking guide of cylindrical section 12is a flat surface.

Cap 18 has a generally round central opening 24 fitted over thecylindrical section 12. The central opening 24 of the cap 18 has aninterlocking guide 26 that mates with the interlocking guide 22 of thecylindrical section 12 and causes the cap 18 to rotate with thecylindrical section 12. Preferably, the interlocking guide 26 of the cap18 is a flat surface.

The upper stop 14 forms the head of the rivet 10 and provides a physicalbarrier which compresses the spring 20 against the slideable cap 18,biasing the cap 18 toward the lower stop 16. A recessed socket 28 iscentrally located on an upper portion 30 of the upper stop 14 and isaligned with a rotational axis 32 of the rivet 10. To rotate the rivet10, a driving apparatus is inserted into the recessed socket 28 of therivet 10.

Referring to FIG. 3, the rivet 10 is shown in use, forming an assembly33 by stir riveting a first workpiece 34, such as a fusible aluminumsheet or plate, to a second workpiece 36, such as a fusible aluminumframe or other substrate. In operation, the rivet 10 is rotated aroundits rotational axis 32.

During rotation, downward force is applied to the rivet 10 causing alower surface 38 of the lower stop 16 to frictionally contact an exposedsurface 40 of the first workpiece 34. The downward force and rotation ofthe rivet 10 cause a portion of the first workpiece 34 to plasticize,allowing the rivet 10 to penetrate the workpiece 34 and create a cavity42. As the rivet 10 is driven through an unexposed surface 44 of thefirst workpiece 34, rivet 10 frictionally contacts an unexposed surface46 of the second workpiece 36. The downward force and rotation of rivet10 cause a portion of the second workpiece 36 to plasticize, allowingrivet 10 to continue penetrating cavity 42. As the rivet 10 is driventhrough the first workpiece 34 into the second workpiece 36, theplasticized material 48 in cavity 42 is intermixed.

Shortly after the lower surface 38 of the rivet 10 penetrates the firstworkpiece 34, the underside 50 of the slideable cap 18 contacts thefirst workpiece 34 to create a seal around the stir site, therebylimiting the amount of plasticized material displaced out of the cavity42. As the rivet 10 advances into the workpieces 34, 36, the cap 18slides up the cylindrical section 12 of the rivet 10, against the forceof spring 20 which forces the cap 18 to press against the firstworkpiece 34. The force of the cap 14 against the first workpiece 34maintains the seal while the cap 18 travels up the cylindrical section12 of the rivet 10. The cap 18 acts as a retaining element, limiting theamount of plasticized material escaping throughout the process.

Upon reaching a desired depth, motion is stopped as shown in FIG. 3 andthe stir site is cooled to harden the plasticized material and providean internally welded joint. The resulting assembly 33 is then heldtogether partially by the mechanical shape of the rivet 10 and partiallyby the welding of the workpieces 34, 36, together with bonding to therivet to form the assembly 33.

Preferably, rivet 10 is driven though the first workpiece 34 andpartially into the second workpiece 36 until the cap 18 of the rivet 10is partially recessed into the exposed surface 40 of the first workpiece34. Thereafter, the rotary motion of rivet 10 is stopped, allowinglocally plasticized material 48 to harden and form several welds. Rivet10 forms a mechanical bond between the first workpiece 34 and the secondworkpiece 36. Plasticized material 48 preferably forms a diffusion bondbetween the rivet 10 and the first and the second workpieces 34, 36.Furthermore, the plasticized material 48 forms an interweld between thefirst workpiece 34 and the second workpiece 36.

The cylindrical section 12 of the body 11 of rivet 10 has a smallerradial thickness than the lower stop 16, to create a re-entrant section52 along the cylindrical section 12. When the rivet 10 is embedded intothe workpieces 34, 36 the re-entrant section 52 extends from the lowerstop 16 up to the underside 50 of the cap 18 when the cap 18 iscompressed against the upper stop 14. Allowing plasticized material 48to fill in between the underside 50 of the cap 18 and the lower stop 16of the rivet 10 increases the strength of the mechanical retentionaround the cylindrical section 12 of the rivet 10.

During the process, the slideable cap 18 restricts oxygen access to therivet 10 by creating a seal between the rivet 10 and the first workpiece34. The reduced oxygen supply around the rivet 10 reduces the formationof oxides on the cylindrical section 12 of the rivet 10, which providesa clean surface to form a bond with the plasticized material 48.Allowing formation of an oxide layer would interfere with bondingbetween the cylindrical section 12 and the plasticized material 48.

FIGS. 4 and 5 show a combination 53 of an alternative embodiment offriction stir rivet 54 with an associated rotary drive and biasingapparatus 55. Rivet 54 includes an elongated body 56 having acylindrical section 58, an enlarged upper stop 60 and a lower stop 62.The upper stop 60 has an angled side 64. A receiver, such as a recessedsocket 66, is located on the upper stop 60. Threads 68, having a longlead, extend longitudinally along the cylindrical section 58 of the body56.

A slideable cap 70 is carried on the threaded cylindrical section 58.Cap 70 has a central opening 72 including threads 74 engaging thethreads 68 of the cylindrical section 58. The cap 70 has an angled side76 mateable with the angled side 64 of the upper stop 60. Duringoperation, the cap 70 slides up the threads 68 of the cylindricalsection 58, rotating slightly until the cap 70 engages the upper stop60.

To rotate the rivet 54, a driver apparatus, including a driver 80 and abiasing device 82, engages the rivet 54. The biasing device 82 retainsand surrounds a portion of the driver 80 and includes a retainer 84,telescoped about a portion of the driver and housing a biasing spring 86within the retainer and compressed between the driver and an end of theretainer.

In operation, the driver 80 engages the receiver 66 located on the upperstop 60 of the rivet 54, while the retainer 84 of the biasing device 82is telescoped arround the upper portion of the rivet and the spring 86,through the retainer, urges the cap 70 towards the lower stop 62. As therivet 54 is driven into the workpiece 88, the cap 70 rotates slightly asit slides up the threads 68 of cylindrical section 58 of the rivet 54,which increasingly telescopes the retainer 84 around the driver 80 andsimultaneously compresses the biasing device 82 against the driver 80.The biasing spring 86, housed inside the retainer 84, continuously urgesthe cap 70 toward the lower stop 62, causing the cap 70 to maintaincontact with the exposed surface 90 of the first workpiece 92. Uponreaching the desired depth, motion is stop as shown in FIG. 5. Oncerotational motion stops, the driver apparatus 55, including the retainer84, and the spring 86, may be disengaged from the rivet 54, leaving therivet 54 fixed in the joined workpieces 88.

The threads 68 along the cylindrical section 58 of the rivet 54, createre-entrant portions 94 along the cylindrical section 58. When the rivet54 is embedded in the workpieces 88, the re-entrant portions 94 receivesome of the plasticized material 96 that fills in between the underside98 of the cap 70 and the lower stop 62 of the body 54 to increase thestrength of the mechanical retention around the cylindrical section 58of the rivet 54.

During the process, the slideable cap 70 restricts oxygen access to therivet 54 by creating a partial seal between the rivet and the firstworkpiece 92. The reduced oxygen supply around the rivet 54 reduces theformation of oxides on the cylindrical section 58 of the rivet 54, whichprovides a clean surface to form a bond with the plasticized material96. Allowing formation of an oxide layer would interfere with bondingbetween the cylindrical section 58 and the plasticized material 96.

While the invention has been described by reference to certain preferredembodiments, it should be understood that numerous changes could be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedisclosed embodiments, but that it have the full scope permitted by thelanguage of the following claims.

1. An Apparatus for friction stir riveting of fusible workpiecestogether at a point of engagement, the apparatus comprising: a rivethaving a body formed of a material with a substantially higher meltingpoint than that of the fusible workpieces and including an elongatedcylindrical section and upper and lower stops at opposite ends of thecylindrical section, the rivet being capable of locally forming a cavityof plasticized material of the workpieces upon rotation and forcing ofthe rivet cylindrical section into the workpieces; a cap having acentral opening surrounding the cylindrical section, the cap beingslideable on the cylindrical section between the stops and configured tosubstantially close an open end of the cavity to restrict the escape ofplasticized material from the cavity and limit the entry of externaloxygen into the cavity; and a driver drivalby engageing the body of therivet during installation of the rivet; and a biasing device actingbetween the cap and the driver and, during rotation and forcing of therivet, biasing the cap against the open end of the cavity.
 2. Theapparatus as in claim 1 wherein the biasing device is a spring and thespring is contained within a retainer and the driver extends into theretainer to rotatably engage the rivet and the spring.
 3. The apparatusas in claim 2 wherein a spring engaging portion of the driver isretained within the retainer to form an integrated driving apparatusincluding the retainer, the spring and the driver.
 4. The apparatus asin claim 1 wherein interacting features of the cap and the cylindricalsection cause rotation of the cap together with rotation of the rivet.